Special Air Filters: Custom Sized
There are circumstances where a HVAC system can have an air filter that is not a standard size. What we define as "standard size" are filters that are mass-produced at certain dimensions (width and length). For example, a 16" x 20" filter is a common (standard) sized air filter. You can get filters custom sized to fit your indoor unit properly. After placed in its housing, a filter should never have any gaps that dirt or debris can pass by to the indoor unit. This will cause dirty blower motors and wheels, heat exchangers (furnaces), and evaporator refrigerant coils (air conditioning or heat pumps). Some filters are slid into a filter rack attached to the unit or in the return ducting. Some filters come in a return filter grille. These grilles have a door that swings out; exposing the filter for replacement. An air filter should fit in the filter rack or grille housing and be easily removed and replaced. You can test this by removing the filter from its housing without snagging or bending. If you ever had an air filter forced out of its housing and into the indoor unit, it was either too dirty or undersized. Being too dirty will cause a proper sized filter to come out of its holder because the force of the indoor blower will cause a dirty filter to collapse; forcing it into the system. An undersized filter will do the same. All filters are 1/2" of the exact dimensions of its housing. So a standard 16" x 20" filter is actually 15-1/2" x 19-1/2". This provides a 1/4" play to replace and remove easily. So remember, if you are looking for a custom sized air filter take the exact measurement of its housing (width and length) and subtract 1/2" from each result to ensure a proper fit and filtration.